This application claims the benefit the copending Netherlands Patent Application entitled xe2x80x9cLongitudinal Reinforced Self-Supporting Capillary Membranes and Their Usexe2x80x9d filed on Nov. 3, 1998, and accorded serial number 1010458, which is entirely incorporated herein by reference.
The present invention relates to self-supporting capillary membranes, to the manufacture of such membranes and to the use of these membranes in separation, filtration and purifying techniques.
Micro, ultra, nano or reversed osmosis membranes are increasingly used in the filtration of suspended particles and solids and in the separation of solutes and liquids, of liquids and liquids and of gases and liquids.
A much used shape of these membranes is a flat shape in which the membrane is laid on a support of for instance polyester. Usually the actual separation membrane is a thin layer of polymer which is made porous, which in itself has insufficient strength to resist the pressures applied. In general such a membrane is only suitable for methods in which the pressure is exerted from the side of the separation layer on the membrane supported by the support. A pressure exerted from the other side mostly would cause the membrane layer to detach from the substrate.
Another shape of a membrane is the tubular or capillary shape. Also in this shape a reinforcement is used to obtain the required strength. An early example of this is given by W. W. Cooper et al in U.S. Pat. No. 3,676,193. The membrane is applied here on a tubular porous knitted support. Cooper et all describe both the arrangement on the outer surface of the support and the arrangement on the inner surface of the support. When the membrane is situated on the outer surface it will be suitable for filtration from the inner surface to the outer surface and when the membrane is situated on the inner surface it will be suitable for filtration from inside to the outside. Besides knitted materials also non-wovens are suitable as supporting layer.
The aforementioned supporting layer also serves to prevent the membrane layer from shrinking during the formation of the membrane, particularly during the formation according to the coagulation process, as a result of which the pore structure formed is disturbed. In U.S. Pat. No. 4,061,821 Hayano describes the prevention of this effect and also the reinforcement of the membrane formed by the reinforcing material. In U.S. Pat. No. 5,472,607 Mail-vaganum describes a comparable supported membrane for filtration from the outer surface.
Without reinforcement the tubular or capillary membranes would not be able to resist the working pressure applied. The reinforcement therefore serves to sufficiently increase the bursting pressure or the collapsing pressure, that is to say the pressure at which the membrane bursts or breaks, depending on the direction of filtration. Usually the reinforcement therefore is a knit or a non-woven in which fibres in various directions are present, but none or very few fibres in the longitudinal direction are present.
When the diameter of the membrane is smaller than approximately 8 mm, it will be possible to manufacture self-supporting capillaries without reinforcing materials which are able to resist the desired working pressures. Such membranes are usually also able to resist the desired pressure from both sides and can therefore be back flushed. A first example of such a membrane is described by Stein et al (J. Api. Polymer Science 20, 2377-2391 (1976) and U.S. Pat. No. 4,051,300). Back flushing is also described by Klein and Schneider (Desalination 41, 263-275 (1982)) and more recently by Wenten et al in U.S. Pat. No. 5,560,828. At the moment several membranes which are able to resist pressures from both sides and therefore can be back flushed, are commercially available.
In practice the filtration is carried out in a module which contains a number of capillaries. A shape often used for a filtration element is a shape in which the capillaries are situated parallel in a tubular housing and in which the capillaries are embedded in a xe2x80x9cpotting materialxe2x80x9d on both sides. Such an element in which the permeate is discharged at the tube ends, is described by Mahon et al in U.S. Pat. No. 3,228,877 and 3,228,876. In U.S. Pat. No. 4,997,564 Herczeg et al describe a type of filtration element in which the permeate is discharged through a central tube.
Although the capillary membranes in themselves have sufficient pressure resistance, problems like capillary rupture regularly occur to the filtration elements described above. Said capillary rupture is the result of the fact that whereas the capillaries are fixated at their ends in the potting material, between those ends they have a certain degree of freedom of movement. Because of the supply and discharge of liquid, forces transverse to the capillary may arise in the filtration elements which give rise to bending forces and tensile forces on the point of the transition of embedding to free membrane. In practice most of the capillary ruptures are therefore found on this point. The bending forces are the result of too large an elongation with a certain lateral load. Non-reinforced capillary membranes made of thermoplastic polymers already have an elongation of some percent at a relatively low load. A bending and a tensile force therefore arise with a lateral force. As these are absorbed by the potting and it is also known that the weakest point of the membrane is situated there (Klein, J. Appl. Pol. Sci 20, 2377-2394 (1976)) the rupture will start at that location. Without elongation lateral movement would be impossible. This effect particularly plays a part in the use of so-called laterally streamed or transversal streamed elements such as described in i.a. H. Futselaar, Thesis Technical University Twente (1993) because the flow here by definition is transverse to the capillaries. NL 1004489 describes a filtration membrane element in which the occurrence of transverse forces on capillary membranes is counteracted by installing one or more distribution tubes each having at least one opening to the membrane compartment, transverse with respect to the capillary membranes. Such a solution can of course not be used in transversely streamed elements.
T. C. Bohrer describes in U.S. Pat. No. 3,494,121 a hollow reinforced composite fibre produced by contacting a plurality of monofilaments e.g. from 2 to 500 or more, preferably from about 25 to 75 monofilaments with a solution comprised of a polymer and an organic solvent to form a unitary filamentary structure and heating said structure to remove the solvent and form a hollow fibre.
In GB 1 374 704 a tubular membrane is disclosed having a wall permeable to liquids and a tubular reinforcing means which is embedded in the membrane material and consists of threads crossing each other, the threads extending in two substantially mutual perpendicular directions, one of the directions being generally axial. As only example of the tubular reinforcing means a tubular, woven fabric is given.
It is an object of the invention to solve the problem of capillary rupture by providing capillary membranes with an increased tensile strength and reduced elongation in longitudinal direction, in which the flow through the membrane is not affected by the presence of the reinforcing fibre or fibres and which are easy to produce.
It was found that by incorporating reinforcing fibres in the wall material of the capillary membrane the ultimate tensile strength of the membrane can be increased without affecting the functioning of the membrane.